class_docstr: Basic tests for Kernel function 
class_docstr_tok: ["Basic","tests","for","Kernel","function"]
docstr: Computes the posterior covariance. 
docstr_tok: ["Computes","the","posterior","covariance","."]
code: def _compute_post_covar(cls, kern, X_tr, X_te):
      """Computes the posterior covariance."""
      K_tr = kern.evaluate(X_tr, X_tr)
      K_tetr = kern.evaluate(X_te, X_tr)
      K_te = kern.evaluate(X_te, X_te)
      post_covar = K_te - K_tetr.dot(np.linalg.solve(K_tr, K_tetr.T))
      post_std = np.sqrt(np.diag(post_covar))
      return post_covar, post_std
  
file_path: dragonfly/dragonfly/gp/unittest_kernel.py
imports: from __future__ import absolute_import
  from __future__ import division
  import numpy
  from .kernel import Kernel, SEKernel, MaternKernel, PolyKernel, CoordinateProductKernel
  from ..utils.ancillary_utils import dicts_are_equal
  from ..utils.base_test_class import BaseTestClass, execute_tests
label_desc: Use image node_id 9 for calling the KernelBasicTestCase obj's underlying member method code with example usage: obj._compute_post_covar(cls, kern, X_tr, X_te) and returns: post_covar,  post_std